The present invention relates to dot matrix printheads and, more particularly, to a print pin driver for a dot matrix printer comprising, a U-shaped core having a pair of parallel spaced legs and formed of a plurality of laminations of a material having the qualities of silicon iron; a magnetic field-forming coil wound about one of the legs; a magnetic flux plate of a material having the qualities of silicon iron disposed across the open end of the U of the core between the legs, the flux plate being attached to the other of the legs and in close-spaced adjacent relationship to the one of the legs, the top surface of the flux plate over the other leg being slightly longitudinally beveled, the flux plate having a bore therethrough over the one of the legs and longitudinally aligned therewith; a longitudinal spring-metal beam disposed along the flux plate, the beam being attached on one end to the beveled top surface, the beam extending out over the flux plate on the other end thereof, the beam including an armature piece of magnetically attractable material disposed within the bore in the flux plate, the beam tapering from adjacent the armature piece to the other end thereof and having one edge forming a reinforcing rib along the tapered portion; and, a print pin carried by the other end of the beam and disposed substantially parallel to the one leg of the core.
Dot matrix printers have gained great popularity in the computer art. They are generally inexpensive and operate at high printing speeds. The printheads comprise a plurality of print pins arranged in a pattern, i.e., rectangular or the like, and each individually activatable to strike from a retracted position to an extended printing position and then return to the retracted position. By activating the print pins according to pre-selected patterns at each character position as the printhead moves across the printing medium with a printing ribbon disposed between the printhead and the printing medium, the desired characters are printed at each character position.
A typical prior art printhead pin driver is shown in simplified form in FIG. 1 and generally indicated therein as 10. Pin driver 10 includes an elongated print pin 12 attached to one end of a spring-biased arm 14. The other end of the arm 14 is attached to a non-moving member 16. An armature piece 18 is attached to the arm 14 to be attracted by solenoid coil 20 when power is applied to the wires 22 of the coil 20. Thus, to activate the pin 12, power is applied to the wires 22 which causes the armature piece 18 to be attracted to the coil 20 as indicated by the arrow 24 which, in turn, moves the arm 14 and pin 12 towards the printing medium 26 as indicated by the arrow 28.
As can be appreciated, since the characters to be printed are the size of normal typewriter characters so as to be a duplication thereof, the pins 12 in the actual printing area of the printhead are clustered close together, moving axially in bores provided therefor disposed in the pre-selected pattern being employed to create characters. Accordingly, the remaining components must be clustered around the printing area with the arms 14 extending into the printing area. Such an arrangement was shown in our above-referenced original patent application Ser. No. 436,950 which is now U.S. Pat. No. 4,531,848. A typical prior art printhead can also be seen with reference to U.S. Pat. No. 3,770,092 of Grim. FIGS. 2 and 3 are simplified drawings of the Grim teachings. As depicted in FIG. 2, Grim employs cylindrical coils 20 arranged in a circular pattern about the printhead face 30 containing the patterned bores 32 therein through which the pins 12 move axially to strike the printing medium. As shown in FIG. 3, the arms 14 of Grim are tapered from adjacent the armature piece 18 to the end carrying the print pin 12. To keep the moving mass at a minimum for various reasons well known to those skilled in the art, the arms 14 are of thin spring metal with the edges at 34 folded up at right angles to form integral flexure resistance into the arms 14.
An alternate prior art approach to a printhead driver is shown in FIGS. 4 and 5, which are simplified drawings of apparatus shown in U.S. Pat. No. 4,461,207 of Helinski. While larger in size and intended for driving a short print "hammer" instead of an elongated print pin, the general principle is the same and the Helinski apparatus could be employed to drive a print pin if desired. As shown in FIG. 4, the print hammer 36 is attached to a leaf spring member 38. The spring member 38 is attached at the end opposite the hammer 36 to the angled top 40 of post 42. In its relaxed position, therefore, it assumes the ghosted position of FIG. 4. A pair of permanent magnets 44 are mounted on spaced posts 46 with a coil 48 disposed between them. The magnets 44 are of sufficient magnetic force that, with nothing else affecting the balance, they attract the spring member 38 to the retracted position shown in FIG. 4. To activate the hammer 36 and cause it to strike the printing apparatus (not shown), an electric current is applied to the coil 48 causing it to create a magnetic field (not shown) counter to the fields of the permanent magnets 44. As a result, the total magnetic field acting on the spring member 38 is insufficient to maintain the member 38 in its retracted position and it moves to the ghosted position of FIG. 4 from its own self-biasing restorative force. When the current is removed from the coil 48, the spring member 38 is once again retracted by the permanent magnets 44.
In FIGS. 5 and 6 of the Helinski patent, he depicts a plurality of his drivers 50 being generally each rectangular in shape and arranged in a side-by-side alignment as would be employed in as so-called "line printer" wherein there is a hammer at each character position across the printing medium. A pair of the Helinski drivers 50 according to the embodiment of his FIGS. 5 and 6 are shown in simplified form in FIG. 5 hereof. The cylindrical coils 48 are mounted below a common non-magnetic "focusing plate" 52 having the spring members 38 mounted thereto and having a plurality of rectangular openings 54 therein disposed over respective ones of the coils 48. To concentrate the releasing counter-magnetic field of the coil on the hammer 36, each spring member 38 has a matching rectangular armature piece 18' attached thereto and disposed within its opening 54.
While not specifically shown in the drawing figures hereof, a third approach similar to that of the Helinski patent (i.e. pushing the print pin instead of pulling it) can be seen with reference to Japanese Pat. No. 56-27365 of Nippon Denshin Denwa Kosha in the name of Tadashi Kodama wherein the spring arms of a dot matrix printhead are arranged radially as in the Grim apparatus with permanent magnets replacing the armature pieces 18 of Grim. With the application of current to the coils in that apparatus, the permanent magnets are repelled by the magnetic field of the coils causing the print pins to strike the printing medium under a positive magnetic force instead of from the restorative self-biasing spring tension of the arms themselves as in Helinski.
As can be appreciated from studying the previous materials of the present applicants and the above-referenced prior art and other related materials well known in the art, there are various factors which are important to dot matrix printhead and pin driver design. For one thing, construction should be inexpensive so that the resultant dot matrix printer can remain inexpensive. The pin drivers should be adaptable to being arranged to occupy minimum space so that the printhead can be kept small. As the number of pins employed in the printing pattern has increased in order to produce better "near letter quality" characters, this latter factor has increased in importance. Minimizing the moving mass and heat production are also factors well appreciated by those designing dot matrix printheads.
The primary factor in any computer-related device is speed. As with any other system, data transfer is only as good as the weakest link. Thus, as personal computers, and the like, have become faster and faster at lower and lower prices, there has been a constant striving to provide the consumer with printers which can print at high speeds while maintaining a low cost for affordability. Unfortunately, employing prior art techniques for printhead and pin driver design, construction, and manufacture, a typical economically priced printhead cannot exceed a reliable refire rate of about 1500 Hz whereas refire rates approaching 3000 Hz would be more desirable.
Wherefore, it is the object of the present invention to provide a dot matrix printhead pin driver and method of manufacture thereof affording an economical pin driver capable of multiple mounting in a small space while affording refire rates approaching 3000 Hz and good heat dissipation capability.